I won’t pretend that I completely understand all the implications of the announced reinterpretation of the Japanese constitution’s article 9, the so-called “peace clause”. But I can guess the reasoning someone well-phrased in geopolitics would give for it.
Since the end of the second world war, eg since the Americans imposed a new constitution on Japan (including article 9), Japan wasn’t of much use in any armed conflict, despite being part of various alliances. Even if a Japanese ship was patrolling the seas beside an ally’s and there was an attack, the conditions to allow Japanese to fire back were so ridiculously complicated it was close to unfeasible.
The US has been trying to phase out its presence from this side of the Pacific for a while now, which, considering the ever-growing power of China is quite alarming for the (small) east Asian states that prospered under the protective wing of the US. The whole point of the reinterpretation of the constitution (something that in itself sounds almost ridiculously roundabout to someone (like me) from a country where the constitution is rewritten on a regular basis at the whims of the ruling party) is rather to make Japan a force to count with when considering the power balance of Asia, instead of just being the part of the supply chain of its allies.
I know just how stupid this can sound to people who grew up in (relatively) peaceful environments. At least for me at first it seemed outright absurd that at the dawn of the 21st century, countries would be still calculating and scheming around power balance based on their armies. After all, just imagine if advanced armies engaged each other head on! Utter and immediate destruction would ensue.
It’s all about guerrilla warfare, and supplying poor boys in the Middle-East with arms so that they can annihilate their “heretic” neighbors with even more brutality than before (though it’s hard to outdo the good old methods, just look at the crucifixions by ISIS). No matter how much China (for example) wanted to expand its territory even further, any direct military assault would all-too-probably trigger an immediate chain reaction in the global net of alliances.
Then why does the military participation of Japan matter? Because while outright military conflict isn’t likely, it’s a whole different problem when it comes to disputed territories, or semi-internal conflicts like the recent events in Ukraine. A crisis like that can very easily arise in the extremely diverse sea of ethnicities in south-east Asia, be it naturally or due to provocation.
That’s where a powerful army, like Japan’s comes into the picture. Japan has had one of the most advanced militaries around the world – even despite its constitutional article 9 that (in its initial interpretation) prohibited the execution of military force except for very limited cases of self-defense. It can provide assistance to its allies in an area where equipment left behind from the Vietnam war is the standard.